I'm just doing some reading, and it mentioned capping the fermenter to carbonate the beer by trapping the remaining co2. Has anyone given it a go? What were the results?
Glass carboys are not rated for pressure, I would definitely not recommend trying it there.
If fermenting or finishing in a metal vessel (like a corny keg), you can use a spunding valve to control the amount of pressure in the keg to force carbonation, similar to actively adding CO₂ to the keg to force carbonate after fermentation.
It's a practice born out of the Rhineheitsghebot about not using sugar to carbonate. Modern practices make it easier to carbonate with no ill effect on the beer, as Denny mentions.